1. Field of the Invention
The invention relates to an apparatus and a method for creating color-calibration characteristic curves and/or process-calibration characteristic curves in a printing system.
In the course of the automation of printing systems, the cooperation between the prepress stage, the printing stage and the quality control stage of modern plants has continuously increased. For this reason, for some time the printing industry has been working to create, in consultation with the various manufacturers of systems which produce both the prepress stage and the printing stage and the associated quality equipment, the standards necessary for this purpose. These standards are intended to ensure that defined interfaces from one unit to the other units located in the processing chain are set up. This has the result that, firstly, the quality requirements on the various units and also on the printed product can be better described and also checked by using the quality features, and, secondly, that it is possible for the components from various manufacturers to be incorporated into the process or to be replaced as appropriate. The technical term used for this is “Cooperation for Integration of Prepress, Press and Post Press” (CIP3).
In this context, it is necessary to perform a so-called process calibration, wherein this procedure currently requires the following steps:    1) Selection of a color management test form, such as IT-8.7, this test form being produced by the prepress stage.    2) Calculation of the color separation data for the printing machine to be calibrated by using a RIP (Raster Image Processor).    3) Exposure of the test form onto the printing plates.    4) Printing of the test form with the printing machine to be calibrated.    5) Measurement of the paper sheet through the use of a suitable measuring instrument.    6) Evaluation of the measured values by using a color management program. This color management program knows the initial data (desired values) for the various test fields of the test form, these data being for example present as LAB values (LAB, L—Lightness, A—redness-greenness, B—blueness-yellowness).    7) Comparison of these initial data with the measured values and determination of a new characteristic curve for the printing process.    8) Supplying the characteristic curves to the prepress stage in order to achieve better results in the future, that is to say results lying closer to the original.
These test forms (IT-8.7) contain almost 1000 different measurement areas, which contain all the possible combinations of printing color rasters and from which it is possible to gather which LAB value produces a specific raster combination in the print. This procedure (steps 1-8) is very complicated, time-consuming and costly and has to be repeated at periodic intervals.
In this context, for a long time it has also been the intention in the printing industry to incorporate the image inspection into the printing process. In addition to the print control strips which are standardized for specific systems and which, outside the printed image, have color information defined in accordance with a strict stipulation, which is then registered by an optical scanning instrument and makes a statement relating to the quality of the printed product, there are systems which take the color information directly from the printed image. European Patent No. EP 0 741 027 B1 relates to a method of localizing errors in the printed image within an extremely short time. For this purpose, through the use of an image registration device, actual image data from the printed product are supplied and, through the use of a comparison circuit, are compared with desired image data from a fault-free subject. The registered printed image is reproduced on a monitor and a faulty inspection area in the printed image, which is reproduced on the monitor, is identified by an overlay frame. The result of the inspection that has been derived from the image detail displayed on the monitor is read into a storage cell of a memory.